The invention relates to the non-destructive testing of tubes and pipes. In particular, it relates to the detection of longitudinal, transverse and oblique defects in tube walls using a phased array ultrasonic system.
A new paradigm was defined in the field of non-destructive testing (NDT) with the introduction of phased array (PA) multi-element ultrasonic technology to upgrade the performance of conventional ultrasonic single-element probe systems. A general description of how phased array technology can be adapted to NDT systems is given in U.S. Pat. No. 5,563,346 with recent examples of applications for the inspection of spherically-bounded materials and turbine blades described in U.S. Pat. Nos. 6,279,397 and 6,082,198 respectively.
Another application is that of the inspection of tubes during production. Here, the xe2x80x9cvirtual rotationxe2x80x9d of ultrasonic beams emitted by PA probes means that the probes themselves do not have to move as a tube being inspected passes through an inspection station. For a non-rotating tube, it is necessary to place PA probes at positions that cover the entire circumference of the tube.
Further advantages result when inspecting tubes or similar pieces (e.g., pipes, rods) using a PA ultrasonic NDT inspection system compared to conventional ultrasonic single-element probe systems. Little maintenance is required because such systems are robust with a relatively simple mechanical design, and what maintenance that is required is straightforward and structured for easy and quick implementation. Further, PA technology permits stationary, multi-element probes to thoroughly inspect a moving tube during manufacture with greater flexibility than conventional designs permit.
Such computer-controlled systems can be reliably and rapidly adapted to meet the requirements of various types and sizes of tubes by simply selecting electronically the software that corresponds to a new tube diameter and wall thickness to be inspected. The parameters affected would include the identification of specific piezoelectric elements in each probe for the formation of each beam, the number of beams per probe and the beam angle of incidence with respect to the outer surface of the tube. Further, the focus point of the PA ultrasonic beams can be adjusted electronically to be closer to the suspected location of defects in the tube walls for a given configuration. This is not possible with single-element probes.
PA technology has the important capability of readily detecting a wide range of defects having longitudinal, transverse and oblique orientations. This occurs because PA probes can electronically scan a much wider inspection zone than single-element probes are able to do. Previous work with single-element ultrasonic probes to broaden the inspection zone of such single-element probes is described in U.S. Pat. Nos. 4,718,277, 5,228,343, 5,473,943 and 5,485,751. It involves grouping single-element probes in clusters wherein the centerlines of the elements of such probes are angled relative to one another such that the beams they emit intersect at a predetermined position in the tube or pipe under inspection. Such single-element probe clusters, however, lack flexibility and the capacity to effect comprehensive inspections for oblique defects precisely because the ultrasonic beams are directed along the only a few discrete angles with respect to the inspected region.
Phased array prior art at R/D Tech Inc. in Quxc3xa9bec City, Canada has been based upon a series of phased array probes that entirely encircle a tube, the multiple elements in each probe being angled as if lying on a conic surface with an included conic apex angle of, for example, 135 degrees. In such applications the inspected tube, while passing longitudinally through the ring of piezoelectric elements along the conic axis of the conic array of probes, has its surface analyzed for defects that are principally transverse or perpendicular to the longitudinal axis of the tube. In such systems each individual phased array beam emitted by a given probe enters the tube wall at a separate entry point. Thus a given portion of the tube wall is not inspected from multiple bearing angles originating from a single probe.
The objects of the invention are, therefore, to improve the way in which tubes are inspected during manufacture by being able to detect defects in a nearly continuous range of multiple orientations (e.g. longitudinal, transverse and oblique) in the wall of a tube; and by allowing the same system, with only minor adjustments (if any), to inspect a range of tube diameters and wall thickness for a given tube material and manufacturing process, or in tubes of differing materials and involving differing manufacturing processes.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
According to the invention in one aspect, a method of non-destructive testing for defects in the wall of tubes or pipes to be inspected is based upon a phased array (PA) ultrasonic probe system. The inspected tube may be rotated continuously while it is moving past an inspection station or the inspection station may be moved with respect to the tube. At the inspection station a plurality of PA beams from each of one or more probes are directed into an entry zone in the form of a spot on the outer surface of the tube wall. The position of the entry zone is fixed with respect to the probes. Said beams arrive at the entry zone with a fixed angle of incidence but from a range of bearing angles along conically oriented paths. Thus, the beams enter an inspection volume within the wall of the tube below the entry zone at a constant inspection angle. Defects within the inspection volume are detected by sensing reflected ultrasonic pulses created by such defects. Sensing may be effected by a circularly deployed array of dual-purpose emitter/receptor piezoelectric elements operating in either a pulse/echo or pitch/catch mode, or by dedicated emitters and receptors.
The invention is directed to a PA ultrasonic probe system that will detect reflecting longitudinal, transverse and oblique defects in the entire three-dimensional volume of the wall of a tube while the tube is moving with both longitudinal and rotational motion with respect to an inspection station. This can also be achieved, for example, by having the tube only rotate and the inspection system moving longitudinally with respect to the tube. By xe2x80x9creflecting defectsxe2x80x9d is meant defects that reflect sound waves either back to the source of such waves or laterally to other sensors.
As a preferred arrangement, a constant inspection angle (i.e., the angle of refraction of the beams in the tube wall) is pre-selected such that it meets the requirements of the inspection. These requirements depend principally on the tube material and the method of tube manufacture and establish an inspection angle that is most likely to detect anticipated defects. Since the inspection angle is constant, it follows that the angle of incidence at the entry zone for the ultrasonic beams emitted by the PA probes will also be predefined and constant.
The conically oriented PA beams are preferably generated by an arc of ultrasonic emitter/receptor elements mounted on a common, conic support surface, each of the elements having an emission face that is positioned to lie on the surface of an imaginary cone in space having a conic axis that is substantially normal to the longitudinal axis of the tube and which passes through the entry zone. The arc is preferably a circular arc. A set of elements occupying a sector portion, or all, of the circle constitutes a xe2x80x9cprobexe2x80x9d. One or more probes may be contained within a module that also contains an acoustic coupling fluid.
Each PA beam is formed by the emission of synchronized ultrasonic waves from a sub-set or group of elements positioned within the arc of a probe. Such sound waves are synchronized to provide, for each respective PA beam, a focal point that is located within the inspection volume that corresponds to the entry zone. The total span of arc or arcs employed is preferably sufficient to ensure the detection of reflections arising from defects within the tube wall that are oriented obliquely within the three-dimensional inspection volume of the tube wall, as well as those oriented longitudinally or transversely. The detection mode of the probe or probes can be either pulse/echo or pitch/catch.
The PA probes are configured, according to one embodiment of the invention, in the form of conical sections with discreet elements side-by-side in a single, 1-dimensional, curved continuum for each probe provided by a conical supporting surface. The cone angle of the sections is a function of the predefined angle of incidence for the ultrasonic beams, which are emitted normally to the face of the probe elements, and are directed to strike the outer surface of the tube under inspection. Since the PA beams are preferably so formed as to strike said outer surface of the tube over a fairly small entry zone, tubes of varying geometries can be inspected using the same inspection set-up. This implies that the distance between the probe faces and the outer diameter of the tube is almost constant, being affected only slightly by the curvature of the various tube sizes. Any such small change in this distance is accommodated using mechanical xe2x80x9cbootsxe2x80x9d, containing or made of acoustic coupling fluid or material, that are adapted to fit tubes of different outer diameters.
Although all the beams must strike the tube in the entry zone, the distance between the probe face and the entry zone is generally not the focal length of the beams. Rather, the focal point of the PA beams is adjusted to be as close as possible to the suspected location of defects within the inspection volume so that very small defects can be detected. The optimum beam configuration is calculated using software in the control system to control the phased array probe elements to place the focal points as near as possible to locations most likely to have a defect.
The preferred angle subtended by each probe (also known as the xe2x80x9coptical aperturexe2x80x9d) depends on the anticipated orientation of defects in the volume to be inspected (e.g., defects having longitudinal, transverse and oblique orientations or only a limited combination of these defects). The larger the optical aperture, the wider the range of defects that can be detected. Most defect orientations could be detected with an optical aperture of 360 degrees.
One embodiment of the invention has two mirror-image probes that occupy sectors of a circle located, facing each other, over the tube for providing clockwise and counterclockwise inspections within the tube wall, viewed in cross-section. This allows detection of longitudinal and obliquely oriented defects in the volume of the tube wall. Alternately, the two mirror image probes, or two additional mirror image probes, may be so oriented with respect to the tube to conduct forward and backward inspections for transverse defects, again including obliques, in the volume of the tube wall.
As already described, the ultrasonic beams emitted by the PA probes preferably strike the entry zone on the outer surface of the tube under inspection at the same angle of incidence. In addition, according to the invention, each beam strikes the entry zone from a different direction (i.e., over a range of bearings) around a line normal to the tube at the entry zone. The range of bearings depends on the optical aperture of the probe; this could be 360 degrees for a fully conical probe. Only PA probes having a conical shape, as described above, are capable of doing this.
The preferred number of beams are selected to match the inspection parameters (anticipated orientation of defects, size of tube, speed of tube passing the inspection station, and the number of inspection modules). The conical shape of the probe allows all of the beams to be readily directed to the same entry zone. A preferred configuration effects inspection of a sample volume with three beams originating from each of two mirror image probes. The high sweep rate of the probes (approximately 15,000 beams/s) is such that the movement of the tube and the entry zone can be neglected for the short period of time it takes for the beams from one sweep to strike it. The rate of sampling for purposes of inspection should be sufficiently high so as to ensure that the entire volume of material in the inspection volume within the tube wall is exposed to PA beams arriving from all of the directions or orientations made available by the probes. In this way, the three-dimensional volume of the tube wall that corresponds to the entry zone during that short time can be inspected for the presence of any longitudinal, transverse or oblique defects, as anticipated by the inspection set-up.
In the next instant, after the tube has moved slightly to expose an adjacent small area of its outer surface as the entry zone, the process is repeated. Thus, as the tube passes through the inspection station, each point on its outer surface momentarily becomes the entry zone. Hence, as the tube advances past the inspection station, a spiral path of inspected tube wall will have passed through an active entry zone. If the tube is advanced at a sufficiently slow rate, this spiral path can cover the entire wall of the tube. Alternately, if the tube is advanced at a higher rate, multiple inspection stations may be progressively deployed along the path of the tube to provide a series of inter-entwined spiral paths of inspected tube wall that include the entire wall of the tube. In this way, complete inspection of the prescribed three-dimensional volume of the tube wall is effected.
In the embodiment of the invention described above an inspection system may consist of one or more pairs of mirror-image probes, each pair contained in a single inspection module. In such a system one pair of probes in a first module may be dedicated to detecting longitudinal defects (including obliques) and a second inspection module may detect transverse defects (also including obliques). Depending on the size of the tube and the speed at which it passes through the inspection station, several identical pairs of inspection modules may be required to detect all of the defects in the three-dimensional volume of the tube wall under inspection. These embodiments are alternates to a single probe having a 360 degree conical section which may replace the pair or pairs of probes with conical sections of lesser span or optical aperture.
Another embodiment of the invention consists of one or more modules each containing a mirror-image pair of PA probes to detect only longitudinal and oblique defects. In this embodiment, transverse defects only (i.e., not including obliques) are detected by two conventional single-element probes, one looking forward and the other backwards along the tube. This embodiment may be further embellished by the addition of one or more single-element probes to simultaneously verify, for example, tube wall thickness or to detect delamination.
In the embodiments described thus far, a PA ultrasonic probe system with a predefined angle of incidence and fixed entry zone is capable of inspecting a range of tube sizes and wall thickness. The only modification that might be required by different tube diameters is changing of the previously-mentioned boot at the interface between the bottom of each module and the outer surface of the tube to ensure the stability of the interface. Such a system may consist of one or more modules.
Should circumstances dictate, a different predefined angle of incidence may require design changes to cater for the altered dimensions of, for example, the conical probe sections and the distance between the probe faces and the outer diameter of the tube. Once built, however, this new system would also be capable of inspecting a range of tube sizes and wall thicknesses without further modification, as described above.
In an additional embodiment, the restriction of having to redesign the module for each predefined inspection angle is overcome by using a 2-dimensional array of ultrasonic probes. In these probes, there are both elements discreetly placed side-by-side lengthwise and around the circumferential surface of the imaginary cone to provide the electronic axis that forms the beams; and elements discreetly placed side-by-side along the inclined surface of the imaginary cone, that allow changes in the direction of the beam, thereby controlling the angle of incidence of the beam with respect to the probe surface. This compares to the 1-dimensional probes of the earlier embodiments that have an electronic axis of variable length but only a single fixed directional axis. Using PA beam steering techniques, the 2-dimensional probes form beams that may be tilted with respect to beams formed with 1-dimensional probes, thereby allowing the angle of incidence to be altered without having to redesign the module.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.